1. Field of the Invention
The present invention relates to a projection exposure apparatus suitably used to manufacture, e.g., a semiconductor integrated circuit, and a large flat panel display.
2. Related Background Art
In a conventional projection exposure apparatus for manufacturing, e.g., a semiconductor integrated circuit and a large flat panel display, one mask (reticle) is held on a predetermined mask holder, and light from a light source is radiated on a photosensitive substrate through the mask and a projection optical system to expose and transfer a pattern formed on the mask to the photosensitive substrate.
In a projection exposure apparatus of this type, the image forming characteristics of the projection optical system are corrected in accordance with the focus state of a mask pattern image formed on the photosensitive substrate, control is performed such that a distance between the photosensitive substrate and the projection optical system becomes constant, and the focus state of a mask pattern image projected on the photosensitive image is optimized.
In such a projection exposure apparatus, a plurality of masks are placed on a mask holder in correspondence with an increase in exposure range on the photosensitive substrate, and the mask holder is moved so that the respective masks are sequentially positioned with respect to the optical axis of the projection optical system. The masks are sequentially exchanged to combine the projected images on the photosensitive substrates, and exposure is performed after the projected images are set to overlap each other.
When a plurality of masks are placed on a movable mask holder and are to be sequentially positioned with respect to the optical axis of the projection optical system while moving the mask holder, the masks placed on the mask holder have different positions along the optical axis. As a result, a conjugate relationship with respect to the projection optical system cannot be kept constant, a projected image on the photosensitive substrate is distorted, and image quality is degraded.
This problem is also posed by a scanning exposure apparatus for scanning a movable mask stage and a substrate stage for holding a photosensitive substrate with respect to a projection optical system. More specifically, a positional relationship (distance) between a mask at a currently scanned position and the photosensitive substrate is changed due to an influence of parallelism between the mask and the photosensitive substrate. Therefore, the conjugate relationship changes.
As one method of solving the above problem, there may be a method of arranging, to the mask holder, an adjusting means for correcting the height (position along the optical axis) and inclination with respect to the optical axis in accordance with the focus state of a projected image. In this case, the arrangement of the projection exposure apparatus is complicated to an extent corresponding to the arrangement of the adjusting means in the mask holder, thus resulting in high manufacturing cost.